Liddell was Headmaster of Westminster School from 1846 to 1855. Meanwhile, his life work, the great lexicon (based on the German work of Franz Passow), which he and Robert Scott began as early as 1834, had made good progress, and the first edition of Liddell and Scott's Lexicon appeared in 1843. It immediately became the standard Greek–English dictionary, with the 8th edition published in 1897.[5]

As Headmaster of Westminster Liddell enjoyed a period of great success, followed by trouble due to the outbreak of fever and cholera in the school. In 1855 he accepted the deanery of Christ Church, Oxford. In the same year he brought out his History of Ancient Rome and took a very active part in the first Oxford University Commission. His tall figure, fine presence and aristocratic mien were for many years associated with all that was characteristic of Oxford life. Coming just at the transition period when the "old Christ Church," which Pusey strove so hard to preserve, was inevitably becoming broader and more liberal, it was chiefly due to Liddell that necessary changes were effected with the minimum of friction.

In 1859 Liddell welcomed the then Prince of Wales when he matriculated at Christ Church, being the first holder of that title who had matriculated since Henry V.[5] While Liddell was Dean of Christ Church, he arranged for the building of a new choir school and classrooms for the staff and pupils of Christ Church Cathedral School on its present site. Before then the school was housed within Christ Church itself.[citation needed]

In July 1846, Liddell married Lorina Reeve (1826-1910), with whom he had nine children including Alice Liddell (1852-1934) of Lewis Carroll fame.

In conjunction with Sir Henry Acland, Liddell did much to encourage the study of art at Oxford, and his taste and judgment gained him the admiration and friendship of Ruskin. In 1891, owing to advancing years, he resigned the deanery. The last years of his life were spent at Ascot, where he died on 18 January 1898.[5] Two roads in Ascot, Liddell Way and Carroll Crescent honour the relationship between Henry Liddell and Lewis Carroll.

The Vice-Chancellor who matriculated me [1872] was the majestic Liddell, who, with his six feet of stately height draped in scarlet, his 'argent aureole' of white hair, and his three silver maces borne before him, always helped me to understand what Sydney Smith meant when he said, of some nonsensical proposition, that no power on earth, save and except the Dean of Christ Church, should induce him to believe it.

A Greek-English Dictionary Based on the German Work of Francis Passow, Oxford: Oxford University Press, 1843, and numerous editions of the same, including abridgments for student use, written with Robert Scott.